1. Field of the Invention
The invention generally relates to a technology for communicating between a tire/wheel assembly and a body of a vehicle. More specifically, the invention relates to a technology for communicating between a wheel-side communication device mounted on a tire/wheel assembly and a body-side communication device mounted on a vehicle body.
2. Description of the Related Art
Systems are known that establish communication between a tire/wheel assembly (hereinafter simply referred to as “wheel”) and a body of a vehicle. One such known system performs communication between a wheel and a vehicle body using a wheel-side communication device mounted on a wheel so that it rotates together with the wheel, and a body-side communication device mounted in a fixed position on the vehicle body.
For example, the system disclosed in JP(A) 10-309914 includes transmitting means mounted to a wheel and receiving means mounted to a vehicle body. The transmitting means detects a tire air pressure of the wheel and transmits a signal indicative thereof, while the receiving means receives the signal sent from the transmitting means and obtains the tire air pressure.
This system also includes a wheel speed sensor mounted on the wheel and determining means mounted on the vehicle body. The wheel speed sensor is fixed in a position near a rotating body, which rotates together with the wheel, and detects the wheel speed. The determining means receives a signal from the wheel speed sensor and determines the tire air pressure state of the wheel based on that signal.
The system further includes a signal wire on the vehicle body, which electronically connects the wheel speed sensor and the determining means together. The signal wire in this system functions as an antenna so that the receiving means can receive the signal sent by the transmitting means. This kind of communication system is also disclosed in JP(A) 11-20427.
With these kinds of technology, the wheel-side communication device is typically mounted in a fixed position on the wheel. As a result, the position of the wheel-side communication device changes as the wheel rotates with respect to the body-side communication device. This change in the relative positional relationship between the wheel-side communication device and the body-side communication device affects the state of communication between the communication devices.
Ideally, a good communication state between the wheel-side communication device and the body-side communication device should be maintained, regardless of the rotational position of the wheel, i.e., the wheel-side communication device. In reality, however, the communication state between the communication devices changes as the relative position between the communication devices changes, as described above.
There are several reasons for this change in the communication state. For example, one reason is that the radio wave transmission path between the communication devices deteriorates as the relative position between them changes due to the fact that the antenna of the communication device is a directional antenna. Another reason is that the distance between the two communication devices changes as the relative position between the communication devices changes. Still another reason is that certain elements of the vehicle temporarily come between the communication devices, at which time these elements act as radio wave interfering objects.
When the communication state between the communication devices changes, the receiving voltage of the communication device may drop below the lowest voltage at which reception is possible during one rotation of the wheel, as shown in the graph in FIG. 15 for example. As a result, there may be a region where communication is poor.